Lasers are known to be important tools for processing a wide range of materials. Example processes include welding, drilling, cutting, routing, perforating, sintering and surface treatment. Materials can include metals, semiconductors, dielectrics, polymers, as well as hard and soft biological tissue. By focusing a beam, it can be possible to achieve improved precision of the laser's action in a direction transverse to the beam axis. However, localizing the laser's action in the axial direction of the beam can be difficult.
Common to many laser processes, are metrology techniques to guide a processing system and obtain quality assurance data before, during and/or after the laser action. Aspects of the laser interaction and practical limitations can interfere with the standard techniques. Some examples of such aspects include plasma generation/electrical interference, high aspect ratio holes, blinding by the processing laser, fast moving material, unpredictable geometries, material relaxation and potential damage to the metrology instrumentation by the processing laser.
Control of laser cut depth is a major enabler for the use of lasers in a variety of microsurgeries. In particular, there exists an enormous demand for spinal surgeries (one third of neurosurgery cases in some hospitals). Current mechanical tools are archaic and difficult to use safely and efficiently except by experienced surgeons. It would be desirable to use lasers because of their high transverse control, no tool wear and non-contact operation (infection control). There are other benefits from laser use such as flexible coagulation control and a natural aseptic effect. However, lasers have very poor axial control (meaning, the beam continues in the axial direction). This means that if the point of perforation is not controlled with extreme precision, unintended injury to surrounding soft tissue is almost certain. Thus, the use of lasers has so far been precluded in a vast number of cases.
Current laser systems are mainly used on soft tissue and rely on an assumption of constant material removed for a given amount of exposure. However, this assumption is not always a good one and furthermore, one often does not know exactly how much tissue needs to be removed a priori. Precision cutting or ablation at interfaces of tissue with vastly different optical, mechanical, and thermal properties is of particular interest to neurological, orthopedic, ear-nose-throat, and laparoscopic surgeons. Unlike corneal laser surgery, these surgical specialties are mainly concerned with nontransparent, optically turbid tissue types with heterogeneous tissue properties on the microscopic scale, where detailed and precise a priori opto-thermal characterization is not feasible. The resultant nondeterministic tissue cutting/ablation process greatly hinders the use of lasers during such surgeries. For example, several authors have recently highlighted that practical laser osteotomy (surgical procedure to cut bone) is limited by a lack of laser depth control. The potential benefit of precise removal of tissue may provide significant clinical impact in this and other areas of surgical oncology and implantation.
In industrial applications, laser processing has the advantage that a single laser can be used to clean, weld and/or machine different materials without mechanical adjustment or changing chemical treatments. Although laser ablation of heterogeneous or multi-layered samples has been accomplished, these processes require tremendous amounts of development and rely on uniform sample characteristics or models with limited applicability and varied success. Laser welding and cleaning, too, typically require extensive multi-parameter optimization. This problem of achieving a specific set of processing objectives (for example feature aspect ratio, heat affected zone, etc.) within the available parameter space (encompassing feed rate, pulse energy, pulse duration, wavelength, assist gas, spot size and focal position) is compounded by characteristics of the material (for example melt and ablation threshold and polymer molecular weight). Accordingly, industrial laser process development requires significant time and financial investment, and may demand fine tolerance feedstock to ensure reliability. Laser process monitoring and control of welding and drilling has used sensors to measure the metal temperature, reflectivity and plasma temperature near the area being processed. These forms of metrology do not provide an accurate measurement of laser beam penetration depth.
Laser welding is an industrial process that is particularly well suited to automated and high volume manufacturing. The diverse applications for laser welding have in common a process of controlled heating by a laser to create a phase change localized to the bond region. Controlling this phase change region (PCR) is important to control the geometry and quality of the weld and the overall productivity of the welding system. The high spatial coherence of laser light allows superb transverse control of the welding energy. Axial control (depth of the PCR) and subsequent thermal diffusion are problematic in thick materials. In these applications, the depth of the PCR is extended deep into the material (e.g. 50 micrometers and deeper) using a technique widely known as “keyhole welding”. Here, the beam intensity is sufficient to melt the surface to open a small vapor channel (also known as a “capillary” or “the keyhole”) which allows the optical beam to penetrate deep into the material. Depending on the specific application, the keyhole is narrow (e.g. <mm) but several millimeters deep and sustained with the application of as much as ˜105 W of optical power. As a result, the light-matter interaction region inside the PCR can be turbulent, unstable and highly stochastic. Unfortunately, instability of keyhole formation can lead to internal voids and high weld porosity resulting in weld failure, with potential catastrophic consequences. Weld quality verification is usually required, often using expensive ex situ and destructive testing. Welding imaging solutions are offered but are limited in their capabilities and usually monitor regions either before or after of the PCR, to track the weld joint, or record the top surface of the cooled weld joint.